Saturday, August 2, 2014

Precursor to a Low-Mass Helium White Dwarf

The Wide Angle Search for Planets (WASP) program has led to
the discovery of several dozen transiting exoplanets. In addition to detecting
transiting exoplanets, the unique capabilities of the WASP program also allows
it to observe various other astrophysical phenomena such as eclipsing binary
star systems where two stars in a binary star system periodically eclipse one
another. An eclipsing binary star system known as WASP 1628+10 is one such
example that was identified from the WASP database. WASP 1628+10 consists of an
A-type star (WASP 1628+10A) and the remnant of a disrupted red giant star (WASP
1628+10B). Both components circle around each other in a tight orbit with an
orbital period of 0.72 days.

A paper by Pierre F. L. Maxted et al. (2014) presents new
spectroscopic observations of WASP 1628+10. These observations allowed the
physical parameters of WASP 1628+10A and WASP 1628+10B to be measured. WASP
1628+10A has 1.36 ± 0.05 times the Sun’s mass, 1.57 ± 0.02 times the Sun’s
radius and ~7 times the Sun’s luminosity; while WASP 1628+10B has 0.135 ± 0.02
times the Sun’s mass, 0.348 ± 0.008 times the Sun’s radius and is roughly as luminous
as the Sun. Additionally, the effective surface temperatures of WASP 1628+10A
and WASP 1628+10B are 7500 ± 200 K and 9200 ± 600 K, respectively.

The new observations confirm WASP 1628+10B is indeed the
precursor of a helium white dwarf (pre-He-WD). Low-mass white dwarfs with less
than 0.35 solar-masses are believed to form from the evolution of binary star
systems. The process generally involves the transfer of mass from a puffed-up
red giant star onto a companion star. Eventually, what remains of the red giant
star is a degenerate helium core. Since it no longer has sufficient mass to
fuse helium into heaver elements, it settles down as an anomalously low mass
white dwarf composed almost entirely of helium. Such a star is known as a
helium white dwarf (He-WD).

Models of the pre-He-WD in WASP 1628+10 indicate that it has
an envelope of hydrogen amounting to no more than ~0.005 solar masses. Episodes
of unstable hydrogen fusion (i.e. shell flashes) are expected to cause WASP
1628+10B to shed its hydrogen envelope as it transitions from a pre-He-WD to a
He-WD. High frequency pulsations have also been observed for both components of
WASP 1628+10. The pulsations are believed to be sensitive to internal process
within the stars and can allow detailed studies to be made of their interiors.
In particular, it permits new observational opportunities for the study of the interior
structure of a low-mass white dwarf.